An aromatic polycarbonate resin is generally produced by a so-called solution method in which an alkali aqueous solution of dihydric phenol and phosgene are allowed to react in the presence of an organic solvent such as methylene chloride. The so-obtained aromatic polycarbonate resin is subjected to a drying step after a granulating step in which it is made into granule by removing the organic solvent from the organic solvent solution of the aromatic polycarbonate resin. As a method of obtaining the granule by removing the organic solvent from the organic solvent solution of the aromatic polycarbonate resin, there is known, for example, a method in which the organic solvent solution of the aromatic polycarbonate resin is brought into contact with hot water or steam to form a flake or a granule (Japanese Patent Publications Nos. 11231/1981, 9843/1985, 9875/1970, 43752/1973 and 122393/1979) or a method in which the organic solvent solution of the aromatic polycarbonate resin is gelled by concentration or cooling to form a granule (Japanese Patent Publications Nos. 21033/1961, 22497/1963, 12379/1965, 9875/1970 and 41421/1972 and Japanese Laid-open Patent Publication 41048/1976). However, the granules (including flakes) obtained by these methods have a large residual organic solvent content, and it is difficult to fully remove the residual organic solvent by ordinary drying.
As a method of removing the above residual organic solvent, there has been proposed a method in which the organic solvent solution of the aromatic polycarbonate resin is distilled by mixing it with hot water having a temperature higher than the boiling point of the residual organic solvent. However, the granule obtained by the above method still contains the organic solvent in an amount of hundreds to thousands ppm. For further reducing the above residual organic solvent, it is required to dry the granule at a high temperature for a long period of time, or pelletize the granule with an extruder equipped with a vacuum-vent. Nevertheless, the organic solvent still remains in an amount of tens to hundreds ppm, and the so-obtained product is not free from its influences on heat resistance, hue and physical properties.
As a method of producing an aromatic polycarbonate resin granule of which the residual organic solvent is small, there is proposed a method in which the organic solvent solution of the aromatic polycarbonate resin obtained by the reaction, or a slurry of an aromatic polycarbonate resin having a residual organic solvent is treated with addition of a nonsolvent or a poor solvent, or a method in which the aromatic polycarbonate resin granule having a residual organic solvent is extracted with a poor solvent (Japanese Patent Publication No. 1298/1980, Japanese Laid-open Patent Publication No. 278929/1988 and Japanese Laid-open Patent Publication No. 6020/1989). In the above methods, the organic solvent is fully removed, but the nonsolvent or the poor solvent remains in a large amount, and it is difficult to fully remove the nonsolvent or the poor solvent by drying at a high temperature for a long period of time, not to mention by usual drying. Further, when the drying is strengthened as described above, not only the operation is complicated, but also the aromatic polycarbonate resin has a decreased molecular weight and a degraded hue and includes foreign matter.
As a method of producing an aromatic polycarbonate resin granule having a large bulk density and a uniform particle diameter, there is proposed a method in which a solid aromatic polycarbonate resin which is not dried is extruded with an extruder externally equipped with a die having a great number of fine orifices (Japanese Laid-open Patent Publication No. 169805/1987). However, this method has a problem in that the residual solvent content in the granule varies.